Patients with systemic mastocytosis have elevated levels of mast cell tryptase in their blood. This mast cell marker will be useful in assessing disease activity. It is also present in the blood for several hours following anaphylaxis and should aid in the evaluation of patients experiencing unexplained syncopal episodes. Gastrointestinal disease is common in patients with systemic mastocytosis. Fasting hyperhistaminemia is associated with gastric hypersecretion and peptic disease. Malabsorption, when present, was usually mild and associated with mast cell infiltrations in the lamina propria of the small intestine. Pediatric mastocytosis was examined and found to usually present in the first 6 months of life. Presenting symptoms did not differentiate cutaneous from systemic forms. Plasma histamines are generally elevated, markedly so in diffuse cutaneous mastocytosis and in children prone to develop peptic disease. Idiopathic anaphylaxis is not a form of "occult" mastocytosis, but rather a separate clinicopathologic entity.